1. Field of the Invention
The present invention relates to surface acoustic wave devices (hereinafter, the term “surface acoustic wave” is abbreviated as SAW) such as SAW resonators, SAW filters, and duplexers, and more particularly, the present invention relates to a SAW device using shear-horizontal waves (hereinafter, abbreviated as SH waves).
2. Description of the Related Art
SAW filters have been widely used for band-pass filters of mobile communication devices and other electronic apparatuses. Such known SAW filters include SAW resonator filters and transversal filters.
An exemplary known SAW resonator filter is configured such that an ST-cut 0° X-propagation quartz substrate having excellent temperature characteristics has an interdigital transducer (hereinafter, abbreviated as an IDT) and reflectors disposed thereon. The IDT and the reflectors are both made from an Al electrode material, and use Rayleigh waves produced by excitation of the IDT.
Another exemplary known SAW filter is configured such that an ST-cut 90° X-propagation quartz substrate has an IDT and reflectors disposed thereon. The IDT and the reflectors are both made from an electrode material such as Ta, W, and Au, and use SH waves produced by excitation of the IDT (refer to Japanese Patent Unexamined Application Publication No. 2000-323956, for example).
The former of the two SAW resonator filters described above, which is provided with Al electrodes on an ST-cut 0° X-propagation quartz substrate and which uses Rayleigh waves, causes the following problems:    (1) difficulty in reducing the filter size because of a small reflection coefficient, resulting in the need for a large number of fingers for reflectors in the SAW resonator filter and in a device such as a SAW resonator; and    (2) large losses due to a small electromechanical coupling coefficient.
On the other hand, the latter of the two foregoing SAW resonator filters, which is provided with electrodes made from a heavy metal such as Ta, W, and Au disposed on an ST-cut 90° X-propagation quartz substrate and which uses SH waves, has the advantages of a large electromechanical coupling coefficient and a large reflection coefficient, and thus the advantage of a reduced filter size.
However, using the latter filter, which is provided with electrodes made from a heavy metal such as Ta, W, and Au, gives rise to problems such as a large variation in the central frequencies of the filter caused by a variation in the width and thickness of the electrodes, and consequently the reject ratio is large. That is to say, since the heavier the metals for the electrodes are, the more sharply the acoustic speeds in the metals vary with respect to a change in the film thickness thereof compared to that in light Al, the latter filter has a problem of a large variation in the central frequencies even when the electrodes made from the heavy metal material have substantially the same variations in width and thickness as those of the corresponding light Al electrodes.
One way to reduce such a variation in the central frequencies is to increase the thickness of the light Al to such a degree that SH waves can be excited. However, it is difficult to increase the film thickness of an IDT film to a degree where SH waves are excited because of the limited thickness of the resist film and a strain in the resist film caused by heat with current thin-film forming technologies.